The field of the invention is systems and method for x-ray imaging. More particularly, the invention relates to systems and method for generating x-ray phase contrast images from x-ray attenuation data.
X-rays have been widely used in medical imaging since their discovery in 1895. These relatively energetic photons interact with tissues that have different attenuating properties to generate contrast for visualization of the difference between tissues. Microscopically, the interaction between x-ray photons and tissue is described by different cross-sections of physical processes such as photoelectric, Compton scattering, and others. In the diagnostic x-ray energy range (10 keV-150 keV), the photoelectric effect is predominant for contrast resolution, while the other physical processes only contribute to degrade image quality. The relative contrast between two tissue types drops quickly with increases in x-ray beam energy. However, when the x-ray beam energy is too low, the deposition of x-ray energy in tissues increases, and harmful radiobiological effects may occur. Thus, a delicate balance between the contrast needed for medical diagnosis and the potential detrimental effects of x-ray deposition must be achieved for x-ray-based medical imaging.
To improve contrast resolution such that the local difference between two neighboring local regions of tissues could be better differentiated in medical diagnosis, x-ray computed tomography (“CT”) was introduced in the 1970s. In this method, projection information is acquired from many different view angles and the local distribution of the x-ray attenuation coefficients is reconstructed from the acquired projection data. The introduction of CT methods significantly enhanced physicians' capabilities in medical diagnosis. However, similarly to the conventional x-ray projection method, the requirement of lowering radiation dose favors to use higher energy beams, but at the cost of a decrease in contrast resolution. In addition to the balance between contrast resolution and radiation dose, when high spatial resolution is needed for visualization of fine structures, the noise level of both x-ray projection and CT imaging increases dramatically. In order to maintain the required contrast-to-noise ratio (“CNR”) for visualization of differences between tissues, a high radiation dose is required to compensate for the increase in noise level.
Since its conception, x-ray imaging has been successfully used in medical diagnosis, but the sole contrast mechanism, x-ray absorption, significantly limits wider and safer applications. An ongoing goal in the use of x-rays in medical applications is to have the lowest possible radiation dose, while maintaining the highest possible spatial and contrast resolution, in addition to having quantitative imaging capabilities.
The limitation of a single contrast mechanism in x-ray imaging has motivated investigators to explore the possibility of multiple contrast mechanisms. As a result of wave-particle duality, x-ray beams can also be viewed as waves. The only difference from the familiar wave examples is that the x-ray wavelength is very short, often on the order of angstroms. Due to their wave nature, x-rays will not only be attenuated when they penetrate through matter, but will also experience distortions in wave-front due to the interaction of the wave with the particles inside of the medium. As a result, a local phase shift will be generated in the x-ray wave after exiting the object. Physically, local phase shifts of x-ray waves are determined by the distribution of local electron density in the object. Namely, when x-rays propagate through matter, the internal structural information of the material is encoded into the corresponding wave-front distortions. Thus, by detecting the wave-front distortion in x-ray waves, one may potentially obtain the structural information of an object. Several methods have been used to extract wave front distortion; these methods are collectively referred to as x-ray phase contrast imaging methods.
Currently, the majority of investigations are still limited to the use of x-ray synchrotron radiations. The experimental results from the synchrotron facilities provide very important insights into two aspects. The first is an understanding of x-ray phase contrast signatures of specific diseased tissues for use in medical diagnosis. The second is the use of nearly ideal coherent x-ray beams to determine relevant physical parameters for future medical devices which would use conventional x-ray tubes. However, to make x-ray phase contrast imaging practical in routine medical practice, the following issues must be addressed.
First, independent of the contrast mechanism used in imaging, x-ray photons must be sufficiently energetic to be able to penetrate through the image object. In preclinical applications, such as tissue specimen imaging, photon energies can be as low as 10 keV for acceptable signal level at the detector plane. However, when a more attenuating object is imaged, such as an entire organ or portion of the human body, the low energy photons will be entirely absorbed. Therefore, it is important to extend the preclinical experimental conditions to higher energy x-ray photons. Ideally, the mean x-ray energy should be on the order of 50 keV for most human clinical applications. In addition to photon energy, it is also important that there is sufficient photon flux in order to achieve the required signal-to-noise ratio (“SNR”) at the detector. Therefore, any potentially useful phase contrast imaging method should use a conventional diagnostic x-ray tube and realistically-sized image objects to further demonstrate the clinical relevance of each proposed phase sensitive imaging method.
Second, currently, for physical phantom or tissue specimen experiments, x-ray radiation dose is not a major concern. One can always increase the x-ray exposure level to increase the SNR to demonstrate feasibility of a given phase contrast imaging scheme. However, in clinical applications, the feasibility of achieving acceptable SNR at a clinically acceptable radiation dose level needs to be demonstrated.
Third, currently, due to limited x-ray source capacity when using conventional x-ray tube sources, the data acquisition time has been prolonged significantly to boost the SNR. While this may be acceptable in preclinical experiments, for routine clinical applications, data acquisition must take within seconds or even sub-seconds, not hours as used in the current laboratory work. In order to address this issue, aside from a more powerful x-ray source and improved detector, it is also important to explore innovative data acquisition and corresponding imaging processing methods to reduce data acquisition time.
Fourth, when x-ray phase contrast imaging is used, it often involves a different requirement on the spatial coherence of the x-ray beams compared with standard x-ray imaging. When a spatially coherent x-ray wave is scattered by a granular structure inside the image object, the coherence will be diminished. As a result, the SNR of the phase contrast mechanism drops, and may even be completely lost. This is in stark contrast with the absorption contrast mechanism which is much more independent of the structure of the image object. Therefore, the phase contrast mechanism should perhaps be combined with conventional absorption contrast for medical diagnosis. Instead of using phase contrast as the sole contrast mechanism in medical diagnosis, a potential new paradigm in the future of x-ray imaging could be multi-contrast imaging, where the complementary information from each contrast mechanism is combined for use in medical diagnosis.
In light of the foregoing drawbacks, there remains a desire to provide systems and methods for generating x-ray phase contrast images that are readily attainable in the clinical setting without requiring specialized hardware systems or increased dose exposure to subjects.